Photovoltaic devices use sunlight to generate clean, reliable and unlimited electric power. The present invention is directed to a set of thin film opto-electronic devices having one or more photosensitive opto-electronically active layers and transparent charge transfer layers. More specifically, it relates to the manufacture of hybrid solar cells and multijunction solar cells designed to provide high solar conversion efficiency, low cost and large-scale manufacturing. Hybrid solar cells combine efficient solar matched inorganic semiconductors with the easy processing of the organic polymers. Such devices can be made with simple, non-toxic and inexpensive materials and versatile device configurations. Multijunction solar cells comprise several stacked cells with decreasing bandgaps to allow multispectral conversion, and thus maximize solar energy conversion efficiency.
Prior art crystalline single pin junction and multi-junction solar cells based on inorganic materials offer high solar to electric energy conversion efficiencies. However, such photovoltaic devices are limited in their ability to provide affordable, high specific power due to their rigidity, brittleness, high stowage volume, high manufacturing cost and complexity. Organic solar cells based on conjugated polymers could be manufactured in high volume like plastics at one-tenth the cost of conventional inorganic photovoltaic cells. However, organic semiconductors are generally poor light absorbers, and have inherently low charge mobilities. Thus, the efficiency for organic solar cell is relatively low. By comparison similarly structured inorganic semiconductor based solar cell efficiency can be an order of magnitude higher. To facilitate charge separation and transport in organic semiconductor based devices, various heterojunction systems have been investigated. U.S. Pat. No. 6,706,962 describes a hybrid device structure that combines an organic absorber with a transparent conducting (TCO) oxide window. To enhance device efficiency the interface of the organic/inorganic layers can be increased by using a TCO. Textured substrate also allows the use of extremely thin layers 23 described by Moller et al, which reduces the diffusion length requirements. Other systems include blends of discotic organic crystals or fullerenes with conjugated polymers and inorganic nanoparticles or nano-rods with conjugated polymers. The efficiency and operation stability for the hybrid or the blend heterojunction systems continue to be far below those of inorganic semiconductor based devices. The discotic organic crystals, fullerenes, and the inorganic nanoparticles such as TiO2 in these nanocomposite blends mainly function as electron conductors and by themselves have wide bandgaps and low absorption efficiency. For certain materials such as CdSe, decreasing the particle size to nano-scale can change the band gap to match the solar spectrum. U.S. Pat. No. 6,884,478 describes the preparation of CdSe nanorods. The nanorods are difficult to align in a direction perpendicular to the electrode surface for effective electron transport. Furthermore, in all cases the counterparts of the nano materials in the composites (p-type conjugated polymers) have bandgaps that do not match the solar spectrum.
This invention describes an alternate device configuration to improve the efficiency of conjugated organic polymers. It combines the conjugated organic polymers with a high efficiency, compatible inorganic absorber. Although the organic materials are poor photon absorbers, they can serve as effective windows for the more efficient inorganic absorbers. Relative to other inorganic solar radiation absorber materials, the copper indium selenide (CIS) chalcopyrite semiconductor offers optimum photovoltaic properties and excellent long-term stability for a variety of solar power applications. The CIS absorber offers proven high efficiency of over 20% for thin-film photovoltaic devices; very high absorption coefficient, high carrier mobility and high carrier diffusion length; radiation hardness and excellent reliability in the aggressive space environment. The state-of-the-art CIS photovoltaic technology uses a CIS alloy absorber, comprising p-copper indium gallium diselenide (p-GIGS). The optimum performance p-GIGS absorber material could lead to an excellent hybrid device if a suitable n-type organic semiconductor material was available to produce efficient p/n heterojunction. However, most semiconducting organic/polymeric materials are p-type, i.e. they become positively charged after losing electrons from their n-conjugated systems (by doping) and are able to conduct holes. A large number of organic/polymeric hole transfer materials are commercially available. In contrast, n-type organic/polymeric materials are very rare.
U.S. Pat. No. 4,601,960 describes the fabrication of n-copper indium selenide (n-CIS) based photoelectrochemical cell and the electrochemical formation of an n/p heterojunction. Over 12% efficiency was demonstrated for n-CIS single crystal cell in a photoelectrochemical configuration. U.S. Pat. No. 5,286,306 extends the n/p heterojunction formation concept described in U.S. Pat. No. 4,601,960 to make a solid-state n-CIS thin-film cell. U.S. Pat. No. 7,560,641 offers a method to further transform the heterojunction into a new lightweight flexible solar cell. It also expands the range of absorber materials by using ordered defect chalcopyrite compounds from the (Cu2Se)(In2Se3)n series, where n=1, 2 . . . n. These compounds are self-stabilizing and consistently n-type. These compounds have ideal solar matched direct bandgap ranging from 1.0-1.3 eV without the need for band gap engineering with extrinsic elements such as Ga or S that are required in the current CIGS absorbers.
The present invention solar cell seeks to continue in part the invention of U.S. Pat. No. 7,560,641 to produce a variety of new solar cell configurations. Accordingly, besides the objectives and advantages of my previous U.S. Pat. No. 5,286,306 and U.S. Pat. No. 7,560,641, the main objectives and advantages of the present invention are based on the design and fabrication methods for new lightweight flexible photovoltaic devices that are efficient, stable and sensitive to a wide region of the solar spectrum.